1. Field of the Invention
The invention relates to an optical system, and in particular relates to an optical system built with electromagnetic-wave sensor.
2. Description of the Related Art
Referring to FIG. 1, when images are projected to a screen 20 by a projector 10, in certain circumstances, some information on the screen 20 must be read, such as a bar code 30. Additional external infrared source 40 and infrared light sensor 50 are needed. It is not convenient to set the external infrared source 40 and infrared light sensor 50.